The present invention relates to a staged gravity settling process for removing contaminants, namely water and particulate solids, from light hydrocarbon diluent-diluted bitumen froth derived from water-based extraction of bitumen from oil sand.
Oil sand, as known in the Fort McMurray region of Alberta, Canada, comprises water-wet, coarse sand grains having flecks of a viscous hydrocarbon, known as bitumen, trapped between the sand grains. The water sheaths surrounding the sand grains contain very fine clay particles. In summary then, oil sand comprises: bitumen; particulate solids (coarse sand and clay xe2x80x9cfinesxe2x80x9d); and water. A sample of oil sand, for example, might comprise 70% by weight sand, 14% fines, 5% water and 11% bitumen. (All % values stated in this specification are to be understood to be % by weight.)
When mixed with hot water, the bitumen will separate from the sand grains and be dispersed into the water phase.
For the past 25 years, the bitumen in McMurray oil sand has been commercially recovered using a water-based process. In the first step of this process, the oil sand is slurried with hot water, steam, usually some caustic and naturally entrained air. The slurry is mixed, for example in a tumbler or pipeline, for a prescribed retention time, to initiate a preliminary separation or dispersal of the bitumen and solids and to induce air bubbles to contact and aerate the bitumen. This step is referred to as xe2x80x9cconditioningxe2x80x9d. The conditioned slurry is then further diluted with hot water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or xe2x80x9cPSVxe2x80x9d). The diluted slurry is retained in the PSV under quiescent conditions for a prescribed retention period. During this period, aerated bitumen rises and forms a froth layer, which overflows the top lip of the vessel and is conveyed away in a launder. Sand grains sink and are concentrated in the conical bottom. They leave the bottom of the vessel as a wet tailings stream containing a small amount of bitumen. Middlings, a watery mixture containing solids and bitumen, extend between the froth and sand layers.
The wet tailings and middlings are separately withdrawn, combined and sent to a secondary flotation process. This secondary flotation process is commonly carried out in a deep cone vessel wherein air is sparged into the vessel to assist with flotation. This vessel is referred to as the TOR vessel. The bitumen recovered by flotation in the TOR vessel is recycled to the PSV. The middlings from the deep cone vessel are further processed in induced air flotation cells to recover contained bitumen.
The hot froths (80-85xc2x0 C.) produced by the PSV and flotation cells are combined and subjected to cleaning, to reduce water and solids contents.
More particularly, it has been conventional to dilute this bitumen froth with a light hydrocarbon diluent, specifically naphtha, to increase the difference in specific gravity between the bitumen and water and to reduce the bitumen viscosity, to thereby aid in the separation of the water and solids from the bitumen. By way of example, the composition of naphtha-diluted bitumen froth typically might have a naphtha/bitumen ratio of 0.65 and contain 20% water and 7% solids.
This diluent-diluted bitumen froth, derived from water-based extraction of bitumen from oil sand, is commonly referred to as xe2x80x9cdilfrothxe2x80x9d.
Separation of the bitumen from water and solids is then carried out. This may be done by treating the dilfroth in a sequence of scroll and disc centrifuges. Alternatively, the dilfroth may be subjected to gravity separation in a series of inclined plate separators (xe2x80x9cIPSxe2x80x9d) in conjunction with countercurrent solvent extraction using added light hydrocarbon diluent.
These prior art centrifuge and IPS techniques for removing water and particulate solids from dilfroth have not been entirely satisfactory. Typically the xe2x80x9ccleanedxe2x80x9d froth, (commonly referred to as xe2x80x9cdilbitxe2x80x9d), may still contain at least 1.5% water and 0.5% solids. These contaminants cause problems in the downstream refinery-type processes used to upgrade the dilbit to produce useful end products. More particularly, the water contains chlorides, which cause corrosion in heat exchangers. The solids plug catalysts. For these reasons, the upgrading sector of these plants have specified that the dilbit should contain  less than 1.0% water and  less than 0.3% solids.
Researchers have long sought to develop a practical and viable alternative process which would reliably produce dilbit having the specified smaller concentrations of water and solids. It would be even more desirable to reduce the contamination to levels in the order of  less than 0.5% water and  less than 0.2% solids. In addition, it would be desirable to achieve this using a system which eliminates the centrifuges, as these are expensive to operate and cause emulsification. However, solutions have been constrained by the following realities:
the clays and asphaltenes in the bitumen have an affinity for each other. They tend to concentrate at water/hydrocarbon interfaces and act to limit coalescence of water droplets into larger globules that would settle rapidly to enable further reduction of water content in the dilbit product;
the loss of bitumen with tails must be minimal, as this is environmentally undesirable and of course reduces oil recovery;
NIB ratio in dilbit should not exceed 0.8; and
given the huge volumes processed in these operations, the equipment used should be simple and reasonably inexpensive to operate and additives, such as demulsifiers, should be used only sparingly.
In accordance with one embodiment of the invention, the following steps are practised in combination:
Dilfroth, preferably having a naphtha/bitumen ratio of 0.5:1 to 0.8:1, is fed into a gravity settler vessel, referred to as the xe2x80x9csplitterxe2x80x9d. The splitter has outlet means for withdrawal of solids and aqueous phase from the bottom and outlet means for overflow of the hydrocarbon phase at the top. The vessel should be enclosed at the top and vapor-tight to prevent escape of diluent. The splitter has a feed inlet, preferably intermediate its ends. The vessel may have a sand rake for moving sand to the central bottom outlet. The dilfroth is temporarily retained (for example 15 to 30 minutes) in the splitter chamber so that the froth settles to form a bottom layer of sand and aqueous middlings, a rag layer and a top layer of hydrocarbons (referred to as xe2x80x9cdilbitxe2x80x9d). Middlings is a mixture comprising mainly water containing some fines and bitumen. An underflow stream of middlings and settled sand, containing some hydrocarbon, (collectively referred to as xe2x80x9csplitter tailsxe2x80x9d), is removed through the bottom outlet. An overflow stream of splitter dilbit is removed through the top outlet. The splitter dilbit comprises hydrocarbons contaminated with, typically 3-5% water and 0.5-2.5% solids. The solids are almost entirely fines. The splitter tails comprise mostly water, typically containing 10-25% solids and 8-20% hydrocarbons;
In an optional or preferred feature, the dilfroth is directly introduced into the splitter middlings layer, beneath the rag layer and above the settled sand. The reason for this is explained below;
In another preferred feature, the feed rate of dilfroth to the splitter, per square meter of horizontal cross-sectional rag or vessel chamber area, is maintained below 6 m3/h of dilfroth for each m2 of rag area. More preferably, the feed rate is maintained at about 4 m3/h or less. Otherwise stated, the hydrocarbons/water interface area loading rate or flux is maintained below 6 m/h, preferably below 4 in/h. It is found that the thickness of the rag layer begins to increase if the flux is high, for example at 8 in/h. As a result, oil loss with the tails increases and/or contamination of the dilbit also increases. The reason for this is explained below;
In another preferred feature, the elevation of the hydrocarbon/middlings interface in the splitter chamber is monitored, for example with a capacitance probe. The rate of introduction of dilfroth and rate of tails removal are controlled in response to the elevation of the interface, so as to maintain separation of the interface from the bottom outlet by keeping the interface at a generally constant elevation. This is controlled so as to preferably maintain the hydrocarbon content in the tails at less than 20%, more preferably less than 15%;
In another preferred feature, the splitter raw dilbit is introduced into a large vapor-tight tank (referred to as the xe2x80x9cpolisherxe2x80x9d) and temporarily retained therein for a prolonged period (relative to the retention time in the splitter). For example, the retention time in the polisher might be in the range of 5 to 24 hours.
In another preferred feature, demulsifier is added to the splitter raw dilbit treated in the polisher. As a result of prolonged settling and the use of demulsifier, water droplets coalesce and settle in the polisher chamber, together with fine solids, to produce a polished dilbit overhead product which may contain less than 1.0% water and 0.3% solids, more preferably  less than 0.5% water and  less than 0.2% solids, and a polisher sludge underflow comprising water and fine solids;
In another preferred feature, the splitter tails are mixed with additional naphtha (diluent) and settled in a vapor-tight vessel referred to as the xe2x80x9cscrubberxe2x80x9d. The scrubber is similar in structure to the splitter. The scrubber naphtha/bitumen ratio is quite high, preferably in the range 4:1 to 10:1. At this high naphtha/bitumen ratio, the naphtha strips residual bitumen from the splitter tails, so that there is produced a scrubber overhead stream which is rich in naphtha and contains most of the residual bitumen. This stream is preferably recycled to the splitter feed to help provide the preferred splitter naphtha/bitumen ratio of 0.5:1 to 0.8:1. The scrubber also produces a scrubber tails underflow which is mainly sand, fines and water, typically containing less than 3% bitumen.
With respect to the foregoing, the following will be noted:
That most of the sand and water originally in the dilfroth are separated in the splitter and report to the splitter underflow, leaving a splitter dilbit product containing fine water droplets which are difficult to coalesce and separate by settlingxe2x80x94however the volume of the splitter dilbit is now considerably reduced relative to the volume of the dilfroth feed. More importantly, virtually all coarse, fast settling solids have been removed from the raw dilbit;
That in the splitter hydrocarbon losses with the tails are found to be  less than 15 wt. %, typically about 4-10 wt. %, of the hydrocarbons originally in the dilfroth feedxe2x80x94in contrast, in the IPS system, between 35-50 wt. % of the original hydrocarbons go into the tails;
That in the polishing step, it is now viable to add demulsifier to the splitter dilbit (reduced in volume and free of sand) and to use prolonged retention time to coalesce and settle out the residual water and fine solids, thereby producing a polished dilbit product that meets the desired specification of less than 1.0% water and 0.3% solids. Because the solids entering the polisher are primarily fine clays, a flat-bottom, large diameter enclosed tank can be used to provide the prolonged settling (for example in the order of 5 to 24 hours) needed to separate the water and fines; and
That in the scrubbing step, a high naphtha/bitumen ratio is used to scrub out residual bitumen in the splitter tails to keep bitumen losses to a very low level. The added naphtha is recycled concurrently to use it efficiently and to help provide the desired naphtha/bitumen ratio in the splitter.
The invention arose from a research program in which the settling behaviour of dilfroth was studied using a glass-walled test circuit. Dilfroth was fed into a glass column splitter through a glass inlet pipe connected with the splitter between its top and bottom ends. The incoming stream of dilute froth was not homogeneous. It comprised easily discernible globes of hydrocarbon, pockets of muddy middlings and grains of coarse sand. As the dilfroth stream entered the vessel chamber, a separation process occurred due to gravity settling. As a result, a lower aqueous phase of middlings and an upper hydrocarbons phase were established. The incoming dilfroth was fed directly into the middlings phase. This middlings phase mainly comprised a suspension of clays in water. The initial separation was rapid (a few seconds). The following actions were observed:
the sand grains (60-150 xcexcm) settled quickly through the middlings to the base of the vessel chamber. The sand and some middlings were continually withdrawn and pumped to a scrubber, as further described below;
pockets of incoming middlings, containing only traces of hydrocarbon, joined the aqueous phase and became part of it; and
the incoming hydrocarbons, present in the form of discrete three dimensional structures, which we referred to as xe2x80x9cleaky sacksxe2x80x9d, floated up through the aqueous phase and collected in a rag layer of other oil sacks at the horizontal interface between the middlings and the layer of hydrocarbons which accumulated above.
The leaky sacks were filled with hydrocarbons and had outer skins formed of sub-micron clay particles. The composite density of the sacks was lower than that of the aqueous middlings (density 1.05-1.1 kg/l) because they would float in the middlingsxe2x80x94but the density of the sacks was greater than that of the hydrocarbon phase above the interface, because they would not float in that phase (density 0.76-0.8 kg/l). Hence the composite density of the hydrocarbon laden sacks was apparently between 0.8 and 1.05 kg/l The density of the clay alone was 2.65 kg/l.
The sacks formed the intermediate rag layer, approximately 100 to 200 mm thick at the interface, between the aqueous and hydrocarbon phases. The sacks in the rag layer did not coalesce into larger ones, although some of them did cluster together. They did not readily burst. They appeared to crowd upwardly into the rag layer.
Yet the sacks did not remain in the rag layer indefinitely. They appeared to penetrate the rag layer and, after residing there briefly (a minute or two), they started moving downwardly through the rag layer and then sank through the middlings to the bottom of the vessel chamber. This meant that the sacks underwent a change in composite density and acquired a density greater than that of the middlings. At the same time, the layer of hydrocarbons above the rag increased in volume and excess hydrocarbon overflowed the vessel. Since there was no input of hydrocarbons to the top layer, other than from the sacks, and since no sacks entered the hydrocarbon phase, it follows that the sacks were leaking hydrocarbons through their permeable clay skins. Hence the expression xe2x80x9cleaky sacksxe2x80x9d.
It is our belief that the rag layer becomes a zone of compression, whereby buoyancy force from the middlings compresses the sacks against the layer of hydrocarbons. As a result of this compression, hydrocarbons within a sack pass through the clay skin and enter the hydrocarbon phase above the rag. It is mostly the uppermost sacks in the rag layer that are partially emptied of hydrocarbon by compression. These sacks increase in composite density and sink to the base of the vessel chamber. However, even though they are denser than the middlings, the descending sacks still contain some hydrocarbons. They are only partially deflated. This is clearly visible in the glass vessel, since their shape is now different. During the initial floating period, a sack is spherical and full. When a sack sinks, it is thin and deflated.
The process of hydrocarbon release from the sacks occurs only at a limited rate. We refer to it as xe2x80x9crate of rag permeabilityxe2x80x9d. That is, there is only a certain volume of hydrocarbon that can be released through a unit area of the rag layer in a unit of time. If the delivery of new sacks to the bottom of the rag layer exceeds the rate of rag permeability, the hydrocarbon release from the sacks becomes the limiting factor and the process stalls gradually. The process of emptying the sacks does not respond well to an increase in the rate of delivery. More sacks enter the rag layer from the bottom than can be emptied by the gentle compression of the layer. The depth of the rag layer therefore grows. This increases the depth of rag that must be penetrated by new sacks and by hydrocarbon released from them. The increased rag depth also hinders the removal of partially emptied sacks from the rag layer. This leads to downward rag build-up with the result that rag is withdrawn by the underflow pump, causing an increase in hydrocarbons loss with the splitter tails.
As a result of considerable experimentation, we have determined a preferred limit of splitter feed rate in m3/h for each square meter of rag horizontal cross-sectional area in the splitter chamber. This flux limit is less than 6 m3/h of splitter feed for each m2 of rag area. More preferably the flux should be less than about 4 m/h. At the high end of flux, the loss of hydrocarbons with the splitter tails begins to increase. For example, at a flux of 8 m/h the loss becomes excessive and may jeopardize the performance of the scrubber. This is because the diluent/bitumen ratio in the scrubber will be reduced.
The splitter operation does not appear to be a perfect process. Some small droplets of water (a few microns in diameter) also make their way into the hydrocarbon phase. The hydrocarbon layer is found to contain small quantities of water (3 to 5%) and clays (1.5 to 2.5%), present in the form of tiny droplets. Microscopic examination indicates that the clays are suspended in water and the surfaces of water droplets are coated with clay particles. The composite droplets resist coalescence and appear very stable. Removal or separation of these micron sized droplets by gravity settling is very slow. However, we have shown that, by prolonged settling, preferably coupled with the addition of known demulsifier chemical, the composite water/clay droplets can be flocculated or coalesced into much larger structures which will settle out of the hydrocarbon phase over a period of hours.
We have also shown that a scrubbing action with a high diluent/bitumen ratio (4:1 to 10:1) is effective to recover residual bitumen from the splitter tails, to reduce the loss of hydrocarbons with the scrubber tails to less than 1.0% bitumen and less than 6% naphtha. Most of the naphtha in scrubber tails can be further recovered by steam stripping in a naphtha recovery unit.
Broadly stated, the invention in one embodiment is a process for cleaning naphtha-diluted bitumen froth (xe2x80x9cdilfrothxe2x80x9d) comprising bitumen and naphtha hydrocarbons contaminated with water and solids, the solids comprising sand and fine clay particles (xe2x80x9cfinesxe2x80x9d), comprising: providing a splitter vessel forming a vapor-tight chamber for gravity settling, said vessel having an overflow outlet at its upper end, an underflow outlet at its lower end and means for feeding incoming dilfroth into the chamber; feeding dilfroth into the chamber through the feed means and temporarily retaining it therein so that the dilfroth separates to form a bottom layer of tails comprising aqueous middlings and sand, said tails containing some hydrocarbons, an intermediate layer of rag comprising water, fines and hydrocarbons collected in discrete three dimensional structures, and a top layer of raw dilbit comprising hydrocarbons containing some water and fines, said middlings combining with the rag and dilbit to create a discernible hydrocarbons/water interface; the feed means being operative to directly feed the incoming dilfroth into the middlings; removing dilbit through the overflow outlet; and removing tails through the underflow outlet, said tails containing less than 20 wt. % of the hydrocarbons in the froth.
Broadly stated, in another embodiment the invention is a process for cleaning naphtha-diluted bitumen froth (xe2x80x9cdilfrothxe2x80x9d) comprising bitumen and naphtha hydrocarbons contaminated with water and solids, the solids comprising sand and fine clay particles (xe2x80x9cfinesxe2x80x9d), comprising: subjecting the dilfroth to gravity settling in a vapor-tight splitter chamber to produce an overflow stream of raw dilbit, comprising hydrocarbons containing water and fines, the proportion of water and fines being small relative to the hydrocarbons, and an underflow stream of splitter tails, comprising aqueous middlings and sand; and subjecting the raw dilbit to gravity settling in a vapor-tight polisher chamber for sufficient time to produce an overflow stream of polished dilbit containing less than 1.0 wt. % water and 0.3 wt. % solids and an underflow stream of polisher sludge.
Broadly stated, in another embodiment the invention is a process for cleaning naphtha-diluted bitumen froth (xe2x80x9cdilfrothxe2x80x9d) comprising bitumen and naphtha hydrocarbons and being contaminated with water and solids, the solids comprising sand and fine clay particles (xe2x80x9cfinesxe2x80x9d), comprising: subjecting the dilfroth to gravity settling in a vapor-tight splitter chamber to produce an overflow stream of raw dilbit, comprising hydrocarbons containing water and fines, the proportion of water and fines being small relative to the hydrocarbons, and an underflow stream of splitter tails, comprising aqueous middlings and sand, said tails containing less than 20 wt. % hydrocarbons; mixing the splitter tails with additional naphtha and subjecting the produced mixture to gravity settling in a vapor-tight scrubber chamber to produce an overhead stream of scrubber hydrocarbons and an underflow stream of scrubber tails; and recycling scrubber hydrocarbons to the splitter chamber.